1. Field of the Invention
The present invention relates to a solar cell of tubular configuration and to techniques for fabricating the same.
2. Description of the Prior Art
In an era of increasing energy consumption, dwindling fossil fuel supplies and concern for the environment, solar radiation represents a potential source of energy which is non-polluting and does not deplete natural resources. The problem is one of efficient, low-cost conversion of sunlight to a readily usable form. Solar photovoltaic conversion offers this possibility, and it is a principal object of the present invention to provide a photovoltaic solar cell for converting sunlight to electrical energy efficiently and economically.
Photovoltaic solar cells per se are known. They have been used with considerable success as a power source in space vehicles where advantage is taken of the high vacuum conditions beyond the earth's atmosphere. For example, cadmium sulfide - copper sulfide photovoltaic heterojunction cells operate without degradation for long periods of time in such space vacuum conditions.
Adaptation of such photovoltaic cells for terrestrial use has several problems. First, if the cadmium sulfide - copper sulfide junction material is exposed to the atmosphere, oxidation and other reactions occur causing relatively rapid degradation of cell performance. Thus hermetic packaging must be provided. But this is compounded by the second problem, which is that very large cell area is required to harness economically significant amounts of energy. Thus while hermetic packaging of small, individual cells of a few square centimeters is readily achieved, extension of such techniques to cell areas of many meters has not been practical. Another object of this invention is to provide a photocell packaging technique permitting large area implementation at low cost.
Solar energy conversion also can be achieved with silicon or other semiconductor junction photovoltaic cells. However, such cells require substantially single crystal semiconductor material for optimum efficiency. The growth of such crystalline material in areas large enough for commercial solar energy conversion has not been achieved. A recent technique called edge-defined film-fed growth offers promise for growth of long ribbons of semiconductor material. A further object of the present invention is to provide techniques for semiconductor photovoltaic cell construction in which oriented semiconductor crystalline growth over large areas is promoted by appropriate surface preparation of the supporting structure.
Another problem of terrestrial solar energy conversion relates to concentration of sunlight onto the cells to obtain maximum efficency. Large flat arrays use only the direct sunlight and do not permit such concentration, and suffer the further disadvantage that replacement of individual cells in the array is difficult. Situating the photocell at the focus of a parabolic reflector provides excellent concentration, but the cell area is severely limited, so that the overall amount of obtained electrical energy is not great. Another object of the present invention is to provide a unique tubular photocell configuration and an associated reflector of line generated paraboloid geometry. This combination permits implementation of large area solar cell arrays having the attendant efficiency gain benefit of light concentration from the paraboloid reflector. Replacement of individual cells, should this be required, is simplified by providing symmetric electrical contacts at each end of the tubular cell.